1. Field of the Invention
The present invention relates to a semiconductor device fabrication method and structure thereof, and relates more particularly to a semiconductor device fabrication method for reducing the defects of dislocation existing in the device.
2. Description of the Related Art
The spectrum of light emitted by the Group III nitride-based semiconductor material ranges from the wavelength of visible light to that of UV light. Furthermore, the Group III nitride-based semiconductor material is a direct transition material so it is widely applied to light emitting diodes (LEDs), laser diodes, light emitting devices, and so on.
Currently, the methods for fabricating high-quality Group III nitride-based semiconductor material devices generally includes growing Group III nitride-based semiconductor material layers on a suitable but imperfect substrate such as a sapphire substrate, a silicon substrate, a GaAs substrate, a silicon carbide (SiC) substrate, or analogous hetero-epitaxial substrates. However, for all such hetero-epitaxial substrates, lattice mismatch and thermal mismatch occur in the process of depositing a high-quality Group III nitride-based semiconductor material layer. The lattice mismatch results from the different intervals between the atoms of crystals, and the thermal mismatch results from the different thermal expansion coefficients of the various materials.
The differences between the lattice constants of silicon carbide and GaN group compound are around 3%. The differences between the lattice constants of sapphire and GaN group compound are around 13%. During the epitaxial process, the lattice mismatch often causes the dislocation problem. That is, the epitaxial device has the defects of line dislocations which are longitudinally (the direction perpendicular to the surface of the substrate) through the crystals. Generally, the Group III nitride-based semiconductor device has line dislocations with a density of 109/cm2. These numerous line dislocations can penetrate each of the Group III nitride-based semiconductor layers composing of different materials up to the top layer. Consequently, the device is cracked by the line dislocations. In view of the aforesaid problems, the threshold current of the laser diodes and the working life and the reliability of the LEDs and the laser diodes are accordingly reduced.
Furthermore, the thermal mismatch should be noticeable. After the Group III nitride-based semiconductor material is grown on the substrate and the device (a sample) is cooled, the differences between the speeds of the thermal contraction of two materials cause high stress. The stress is directly related to the thickness of the deposited film layer, and, particularly, is directly proportional to the thickness. For example, the thermal expansion coefficient of sapphire is greater than that of GaN. When a sapphire substrate and a GaN layer are cooled simultaneously, the thermal mismatch between them causes compression stress on the GaN layer and tensile stress on the sapphire substrate. When the thickness of the layer is larger than 10 μm and the GaN layer is unable to withstand the compression stress, cracks are likely to occur in the layer.
The broad defects (line dislocations, and misfit dislocations) existing in the device dramatically and accordingly reduce its efficiency and working life. In particular, the occurrence of the dislocation seems to result in the center of the non-radiative recombination so the lighting efficiencies of LEDs and laser diodes composed of these materials are reduced. Furthermore, the occurrence of the dislocations also increases dark current. Even though the problem of the line dislocations does not block the development of the extremely bright LED, the dislocations cause p-n junction devices such as transistors with high electronic mobility, field effect transistors (FET), and other similar devices to have excessive reversely biased leakage current. The occurrence of the dislocation results in the strong light scattering center of carriers so that the mobility of the electrons and holes is reduced. The performance of various semiconductor devices is limited by the dislocations.
U.S. Pat. No. 6,534,332 discloses a method of manufacturing a GaN thin film. As shown in the schematic diagram of a structure 100 of FIG. 1, the detail method is as follows: Under an epitaxial circumstance at a high temperature (larger than 950° C.), a first GaN layer 150 is grown on a buffer layer 130 on which is grown on a substrate 110 at a low temperature. Under an epitaxial circumstance at a middle temperature of between 700° C. and 900° C., a GaN middle layer (IT-IL) 170 is formed, and a second GaN layer 190 is grown on the GaN middle layer 170 at a high temperature. The objective of the prior art utilizes the GaN middle layer 170 which is formed at middle temperature epitaxial conditions to improve the epitaxial quality. However, the buffer layer 130, the first GaN layer 150 and the GaN middle layer 170 are all GaN group materials. The dislocations of the bottommost layer easily penetrate the GaN middle layer 170 to the interior of the device, and accordingly, the defects cannot be effectively eliminated.
Furthermore, U.S. Pat. No. 7,135,716 provides an LED. The LED is characterized in that a polarity conversion layer is formed in the LED. However, the polarity conversion layer is disposed on an amorphous buffer layer so the density of the defects in the material cannot be effectively reduced. The prior art utilizes (AlxInyGaz)Mg3-(x+y+z)N2 and SiaMg3-aN2 as the II-III group Nitride or II-IV group Nitride. These materials differ from the III group nitride compound material. The growing condition is more complicated and severe, and is unfavorable to the manufacture.
In view of above, a new structure or a new manufacturing process of a semiconductor device needs to be developed. Accordingly, the objective to reduce the internal defects of the semiconductor device can be achieved. The yield of the process would be improved, and the reliability and working life of the device would be increased. The market of photoelectric devices will be satisfied by the development of such method.